In LTE (Long Term Evolution) Advanced of 3GPP (Third Generation Partnership Project), OFDMA (Orthogonal Frequency Division Multiplexing Access) using MU-MIMO (multi-user multiple-input multiple-output) has been proposed. In MU-MIMO downlink transmission, a base station is able to not only communicate with multiple mobile communication terminals, but also to transmit different data streams (layers) simultaneously to a mobile communication terminal.
In addition, in LTE Advanced, a reception technique for mobile communication terminals called as interference rejection combining has been discussed. Interference rejection combining (IRC) is a technique for downlink communication, in which a mobile communication terminal gives weights to signals obtained by reception antennas so as to suppress interference to the desired electric wave beam from the visited base station (desired base station) caused by interfering electric wave beams from interfering base stations at the mobile communication terminal. IRC improves the reception quality of desired signals on a desired electric wave beam especially in a case in which a mobile communication terminal 10 is located near the boundary of a visited cell 1a (cell of the desired base station 1) and receives strong interfering electric wave beams from another base station 2 (interfering base station) as shown in FIG. 1. In FIG. 1, reference symbol 2a denotes the cell of the interfering base station 2. In addition, in FIG. 1, a general shape of a beam 1b generated at the desired base station 1, and a general shape of the beam 2b generated at the interfering base station 2 are illustrated. A part of the beam 2b generated at the interfering base station 2, i.e., a part of a beam for downlink channels for other mobile communication terminals (for example, a mobile communication terminal 12) causes an interfering signal for the mobile communication terminal 10.
IRC is described in, for example, Patent Document 1, Non-patent Document 1, and Non-patent Document 2.
In an IRC reception technique, reception weights WMMSE, i for a mobile communication terminal that receives signals can be calculated with the use of Equation (1) below derived from an MMSE (minimum mean-square-error) algorithm.
                              W                      MMSE            ,            i                          =                                                            P                s                            ⁡                              (                                                      H                    i                                    ⁢                                      W                                          TX                      ,                      i                                                                      )                                      H                    ⁢                                    (                                                                    ∑                                          i                      =                      1                                                              N                      UE                                                        ⁢                                                                                    P                        s                                            ⁡                                              (                                                                              H                            i                                                    ⁢                                                      W                                                          TX                              ,                              i                                                                                                      )                                                              ⁢                                                                  (                                                                              H                            i                                                    ⁢                                                      W                                                          TX                              ,                              i                                                                                                      )                                            H                                                                      +                                                      σ                    i                    2                                    ⁢                  I                                            )                                      -              1                                                          (        1        )            
Equation (1) can be utilized in a case in which information on all downlink channels that may cause large interference can be estimated. In Equation (1), suffix i in each parameter denotes the number of the mobile communication terminal. In Equation (1), Ps is a scalar indicative of a transmission power per symbol from the desired base station for a mobile communication terminal #i. Hi is a channel matrix (channel impulse matrix) of the mobile communication terminal #i. In this channel matrix, the number of rows is the number of receiving antennas of the mobile communication terminal #i, whereas the number of columns is the number of transmitting antennas of the desired base station for the mobile communication terminal #i. In summary, this channel matrix is a channel matrix of downlink channels to the mobile communication terminal #i from the desired base station for the mobile communication terminal #i. WTX, i is a precoding matrix generated at the desired base station for the mobile communication terminal #i and used for downlink transmission from the desired base station to the mobile communication terminal #i. This precoding matrix has rows of which the number is the number of transmitting antennas of the desired base station for the mobile communication terminal #i, and columns of which the number is the number of the transmission layers, i.e., the number of the transmission streams transmitted from the desired base station for the mobile communication terminal #i. If the number of transmitting antennas of the base station is one, this precoding matrix is a scalar. (With this respect, Equation (1) can be also used in SIMO (single-input multiple-output).)
σi2 indicates a noise power at the mobile communication terminal #i, and σi is a standard deviation of the noise power. I is an identity matrix.
NUE is the sum of the total number of mobile communication terminals that receive downlink channels that may significantly interfere with the downlink channel received by the mobile communication terminal for which the receiving weights are to be calculated, and 1 (the number of mobile communication terminal for which the receiving weights are to be calculated). “Downlink channels that may significantly interfere with the downlink channel received by the mobile communication terminal” mean downlink channels that use the same frequency as that for the desired downlink channel.
Superscript H on the right side in the Equation (1) denotes complex conjugate transpose.
According to Equation (1), each mobile communication terminal can calculate receiving weights WMMSE, i not only on the basis of the channel matrix of the downlink channel from the desired base station for the mobile communication terminal to the mobile communication terminal, and the precoding matrix generated at the desired base station for the mobile communication terminal, but also on the basis of channel matrices of downlink channels of signals coming into the mobile communication terminal transmitted from desired base stations for other mobile communication terminals in order to send the downlink signals to other mobile communication terminals, and precoding matrices generated at other base stations for other mobile communication terminals for downlink transmission to other mobile communication terminals. In SU-MIMO, desired base stations for other mobile terminals are different from the desired base station for the mobile communication terminal for which receiving weights are to be calculated.
The mobile communication terminal can estimate the product of the channel matrix of the downlink channel from the desired base station to mobile communication terminal and the precoding matrix thereof, on the basis of a reference signal, which will be described later. In order to use Equation (1), the mobile communication terminal should know or estimate channel matrices and precoding matrices with respect to signals transmitted to other mobile communication terminals, i.e., interfering signals. However, use of Equation (1) enhances the ability to suppress interference by beams from other base stations, because Equation (1) uses channel matrices and precoding matrices with respect to other mobile communication terminals for calculating the receiving weights.
In a case in which information on all downlink channels that may cause large interference cannot be estimated, it is possible to calculate reception weights WMMSE, i for a mobile communication terminal that receives signals by using Equation (2) below, as an alternative IRC reception technique.WMMSE,i=Ps(HiWTX,i)H(Ryy,i−1)T  (2)
In Equation (2), superscript T indicates transposition. Ryy,i is a covariance matrix of the received signal for the mobile communication terminal #i, and is calculated from Equation (3).
                              R                      yy            ,            i                          =                              1            M                    ⁢                                    ∑                              m                =                1                            M                        ⁢                                                            y                  i                                ⁡                                  (                  m                  )                                            *                                                y                  i                                ⁡                                  (                                      m                    T                                    )                                                                                        (        3        )            
In Equation (3), yi(m) is a vector of a signal received at the mobile communication terminal #i, in which m is the sample number of the received signal. Mobile communication terminal #i calculates the received signal vector (yi(m) for each of receiving antennas of the mobile communication terminal #i. The sample number m is a combination of the number of the received subcarrier and the symbol number of the OFDM (Orthogonal Frequency Division Multiplexing) symbol. In Equation (3), the asterisk denotes conjugate, whereas T denotes transpose. M is the total number of samples used for averaging, and is freely determined. Thus, each mobile terminal processes vectors of samples of signals at respective receiving antennas of the mobile communication terminal, and averages the matrices obtained by the process, thereby obtaining the covariance matrix Ryy,i.
According to Equation (2), each mobile communication terminal can calculate the receiving weights WMMSE, i from the channel matrix of downlink channels from the desired base station for the mobile communication terminal to the mobile communication terminal, the precoding matrix for the mobile communication terminal generated at the desired base station, and received signal vectors. It is possible to estimate the product of the channel matrix of downlink channels from the desired base station to the mobile communication terminal and the precoding matrix thereof on the basis of the reference signal, which will be described later. Therefore, if Equation (2) is used, it is unnecessary to estimate channel matrices of interfering waves coming from interfering base stations for downlink signal transmission to other mobile communication terminals. However, for enhancing the ability to suppress interference by beams from other base stations, it is necessary to prepare many samples m used for averaging in Equation (3).
Accordingly, if Equation (1) can be used, the method for calculating receiving weights with the use of Equation (1) is more preferable than the method with the use of Equation (2).
3GPP LTE-Advanced (Release 10) regulates the UE-specific reference signal as a reference signal for demodulation transmitted in the physical downlink shared channel (PDSCH), as described in Non-patent Document 3. Using the UE-specific reference signal, the mobile communication terminal can modulate signals where at most 8 layers (ranks) are multiplexed.
FIGS. 2 through 4 show mapping of the UE-specific reference signals on a resource block. In FIGS. 2 through 4, reference symbol RB designates a single resource block, and each square designates a resource element that is a minimum resource unit defined by a single subcarrier and a single OFDM symbol. Reference symbol RS designates the UE-specific reference signal. The ordinate denotes frequency (subcarrier), whereas the abscissa denotes time (OFDM symbol).
FIG. 2 shows mapping of the UE-specific reference signals in a case in which the number of layers is two. As shown in FIG. 2, the UE-specific reference signal RS of layer L1 uses three subcarriers, and uses two consecutive OFDM symbols periodically (intermittently). Similarly, the UE-specific reference signal RS of layer L2 uses three subcarriers that are also used for layer L1, and uses two consecutive OFDM symbols periodically (intermittently). The UE-specific reference signal RS of layer L1 is distinguished from that of layer L2 by means of different orthogonal codes of which the sequence length is two. That is to say, with the use of code divisional multiplexing (CDM) by using different orthogonal codes, the UE-specific reference signals RS for layers L1 and L2 are distinguished from each other. In other words, two layers L1 and L2 are orthogonally multiplexed. Thus, twelve resource elements per resource block RB are used for the UE-specific reference signals RS.
FIG. 3 shows mapping of the UE-specific reference signals in a case in which the number of layers is four. As shown in FIG. 3, the UE-specific reference signal RS of each layer (layer L1, L2, L3, or L4) uses three subcarriers, and uses two consecutive OFDM symbols periodically (intermittently). The UE-specific reference signals RS for layers L1 and L2 use the same subcarriers, but are distinguished by means of different orthogonal codes of which the sequence length is two. That is to say, with the use of code divisional multiplexing (CDM) by using different orthogonal codes, the UE-specific reference signals RS for layers L1 and L2 are distinguished from each other. The UE-specific reference signals RS of layers L3 and L4 are sent by subcarriers that are different from those for the UE-specific reference signals RS of layers L1 and L2. That is to say, layers L3 and L4 are distinguished from layers L1 and L2 by means of frequency division multiplexing (FDM). The UE-specific reference signals RS for layers L3 and L4 use the same subcarriers, but are distinguished by means of different orthogonal codes of which the sequence length is two. That is to say, with the use of code divisional multiplexing (CDM) by using different orthogonal codes, the UE-specific reference signals RS for layers L3 and L4 are distinguished from each other. Consequently, four layers L1, L2, L3, and L4 are orthogonally multiplexed. Thus, 24 resource elements per resource block RB are used for the UE-specific reference signals RS.
FIG. 4 shows mapping of the UE-specific reference signals in a case in which the number of layers is eight. As shown in FIG. 4, the UE-specific reference signal RS of each layer (each of layers L1 through L8) uses three subcarriers, and uses two consecutive OFDM symbols periodically (intermittently). The UE-specific reference signals RS for layers L1, L2, L5, and L7 use the same subcarriers, but are distinguished by means of different orthogonal codes of which the sequence length is four. That is to say, with the use of code divisional multiplexing (CDM) by using different orthogonal codes, the UE-specific reference signals RS for layers L1, L2, L5, and L7 are distinguished from one another. The UE-specific reference signals RS of layers L3, L4, L6, and L8 are sent by subcarriers that are different from those for the UE-specific reference signals RS of layers L1, L2, L5, and L7. That is to say, layers L3, L4, L6, and L8 are distinguished from layers L1, L2, L5, and L7 by means of frequency division multiplexing (FDM). The UE-specific reference signals RS for layers L3, L4, L6, and L8 use the same subcarriers, but layers L3 and L4 are distinguished by means of different orthogonal codes of which the sequence length is four. That is to say, with the use of code divisional multiplexing (CDM) by using different orthogonal codes, the UE-specific reference signals RS for layers L3, L4, L6, and L8 are distinguished from each other. Consequently, eight layers L1 through L8 are orthogonally multiplexed. Thus, 24 resource elements per resource block RB are used for the UE-specific reference signals RS.
The UE-specific reference signal can indicates the product HiWTX, i of the channel matrix Hi of downlink channels to the mobile communication terminal that receives the UE-specific reference signal for demodulating received signals and the precoding matrix WTX, i for the mobile communication terminal.
The mobile communication terminal transmits the channel matrix Hi of the downlink channel calculated at the mobile communication terminal to the desired base station. The desired base station calculates the precoding matrix WTX, i for the mobile communication terminal from the channel matrix, and informs the mobile communication terminal of the product Hi WTX, i of the channel matrix and the precoding matrix by means of the UE-specific reference signal.
As described in Subclause 6.10.3.1 of Non-patent Document 3, the UE-specific reference signal is scrambled by a scramble sequence r(m) at each cell (i.e., each base station) in addition to the orthogonal code of which the sequence length is 2 or 4. The scramble sequence r(m) is allocated depending on the subcarrier number m of the subcarrier on which the UE-specific reference signal is sent. The scramble sequence r(m) is calculated in accordance with Equation (4) below.
                              r          ⁡                      (            m            )                          =                                            1                              2                                      ⁢                          (                              1                -                                  2                  ·                                      c                    ⁡                                          (                                              2                        ⁢                        m                                            )                                                                                  )                                +                      j            ⁢                                                  ⁢                          1                              2                                      ⁢                          (                              1                -                                  2                  ·                                      c                    ⁡                                          (                                                                        2                          ⁢                          m                                                +                        1                                            )                                                                                  )                                                          (        4        )            
In Equation (4), m is the subcarrier number, and in a case in which a normal cyclic prefix is used, m is a variable obtained by the following equation:m=0,1, . . . ,12NRBmax,DL−1.In a case in which an extended cyclic prefix is used, the subcarrier number m is a variable obtained by the following equation:m=0,1, . . . ,16NRBmax,DL−1.
NRBmax,DL is the largest downlink bandwidth configuration, i.e., the total number of resource blocks available at the base station.
c(i) is a pseudo-random sequence, of which the initial value cinit is calculated in accordance with Equation (5).cinit=(└ns/2┘+1)·(2NIDcell+1)·216+nSCID  (5)
In Equation (5), ns is the slot number within the radio frame, NIDcell is the physical layer cell identity, and nSCID is the identification number of the scramble sequence and is 0 or 1.
As described above, since the base station scrambles the UE-specific reference signals, the mobile communication terminal should recognize the UE-specific reference signals by descrambling received signals before executing channel estimation using the UE-specific reference signal. Accordingly, the base station informs the mobile communication terminal of the cell ID and the scramble sequence identification number nSCID.
In radio communication systems complying with LTE Release 10, different cell IDs are allocated to individual cells, i.e., individual base stations. For example, in the structure of FIG. 5, cell IDs 1, 2, and 3 are allocated to base stations 1, 2, and 3 (thus, cells 1a, 2a, and 3a), respectively. In FIG. 5, the base station 1 is the desired base station for the mobile communication terminal 10. The mobile communication terminal 10 is located near the boundary of the cell 1a of the desired base station 1, especially, near cells 2a and 3a of interfering base stations 2 and 3, and may receive beams 2b and 3b from the interfering base stations 2 and 3. A part of the beam 2b that is the beam for a downlink channel to another mobile communication terminal (for example, the mobile communication terminal 12) causes interfering signal 2c for the mobile communication terminal 10, whereas a part of the beam 3b causes the interfering signal 3c for the mobile communication terminal 10.
The mobile communication terminal 10 shown in FIG. 5 is informed of the cell ID of the desired base station 1, the number of transmitting antennas of the desired base station 1, the number of transmission layers transmitted from the desired base station 1 to the mobile communication terminal 10, the UE-specific reference signal layer number corresponding to the number of transmission layers, and the scramble sequence identification number nsCID by control signals from the base station 1. The UE-specific reference signal layer number indicates which layer is used for transmitting the UE-specific reference signal that is used for demodulation of signals received by the mobile communication terminal 10 (see FIGS. 2 through 4). By knowing the UE-specific reference signal layer number, the mobile communication terminal 10 can be aware of the subcarriers (frequencies), the OFDM symbols, and the orthogonal codes used for transmission of the UE-specific reference signal. Furthermore, by knowing the cell ID and the scramble sequence identification number nSCID, the mobile communication terminal 10 can calculate the scramble sequence r(m), and can recognize the UE-specific reference signal.
On the other hand, for radio communication systems complying with LTE Release 11, use of a single cell ID of particular multiple cells is under consideration. This is shown in FIG. 6. The radio communication system shown in FIG. 6 includes a remote radio head (RRH) 4 in addition to the structure shown in FIG. 5. The base stations 1, 2, and 3 shown in FIG. 6 are macro base stations each of which transmission power is great and the cell is broad (the cell is a macro cell). On the other hand, a remote radio head is a small base station connected to a macro base station via cable of which transmission rate is high, such as optical fibers, and has little transmission power so that its cell is narrow. The remote radio head is synchronized with the macro base station to which it is connected via cable, and the remote radio head itself communicates with mobile communication terminals. To the remote radio head, a cell ID that is the same as the cell ID allocated to the macro base station to which the remote radio head is connected via cable is allocated. In FIG. 6, the remote radio head 4 is connected to the macro base station 1 via cable, and synchronized with the macro base station 1. To the remote radio head 4, the cell ID allocated to the macro base station 1 is allocated. Remote radio heads are provided for performing inter-cell coordinated transmission (Coordinated Multi-Point transmission and reception, CoMP). Downlink CoMP is a scheme in which multiple base stations cooperate for signal transmission to a mobile communication terminal.
The mobile communication terminal 10 shown in FIG. 6 is located in the cell 4a of the remote radio head 4. In this case, the remote radio head 4 is the desired base station for the mobile communication terminal 10, and the mobile communication terminal 10 may receive the beam 1b from the interfering base stations 1. A part of the beam 1b that is the beam for a downlink channel to another mobile communication terminal (for example, the mobile communication terminal 12) causes interfering signal 1c for the mobile communication terminal 10.
The mobile communication terminal 10 shown in FIG. 6 is informed of the cell ID of the remote radio head 4 that is the desired base station, the number of transmitting antennas of the remote radio head 4 that is the desired base station, the number of transmission layers transmitted from the remote radio head 4 that is the desired base station to the mobile communication terminal 10, the UE-specific reference signal layer number corresponding to the number of transmission layers, and the scramble sequence identification number nSCID used in the mobile communication terminal 10 by control signals from the remote radio head 4 that is the desired base station.
For the mobile communication terminal 12 located in the cell 1a outside the cell 4a, the macro base station 1 is the desired base station. The mobile communication terminal 12 may receive the beam from the remote radio head 4 that is the interfering base station for the terminal 12, and a part of the beam may cause an interfering signal for the mobile communication terminal 12. The mobile communication terminal 12 is informed of the cell ID of the macro base station 1 that is the desired base station, the number of transmitting antennas of the desired base station 1, the number of transmission layers transmitted from the desired base station 1 to the mobile communication terminal 12, the UE-specific reference signal layer number corresponding to the number of transmission layers, and the scramble sequence identification number nSCID used in the mobile communication terminal 12 by control signals from the desired base station 1. The cell ID of the macro base station 1 is the same as that of the remote radio head 4.